1. Technical Field
The present invention generally relates to an enclosure and mounting apparatus for an acoustic signal generating device. More particularly, the present invention relates to a pressure plate for mounting a loudspeaker within an enclosure for use with an active noise cancellation muffler system.
2. Discussion
The application of active noise cancellation (ANC) technology to eliminate various noise signals is generally known within the art. ANC technology is currently used in a variety of applications including controlling noise produced by industrial blowers, lowering the noise levels within cabins of aircraft, and significantly reducing the noise levels emitted from exhaust systems of combustion engines. These systems typically operate by creating an anti-noise signal which is equal in amplitude and opposite in phase with the primary noise signal. In theory, when the primary noise signal and the anti-noise signal are acoustically combined, the two signals effectively cancel one another which significantly reduces the production of any sound. The ANC muffler systems used with combustion engine exhaust systems are typically either built around the primary exhaust conduit, or are placed adjacent to the primary exhaust conduit. The primary or exhaust noise signal and anti-noise signal are then combined for cancelling the production of sound.
In many ANC muffler systems, a loudspeaker is housed in an enclosure which increases its efficiency, tunes its response range, and protects it from the elements within its operating environment. A variety of enclosure designs have been implemented over the years, two of which are a second order enclosure and a fourth order enclosure. The second order enclosure can take on several shapes, but in its simplest form, is a housing with a loudspeaker mounted inside on an open face. The dimensions and volume of the box or enclosure work in conjunction with the loudspeaker characteristics to determine the output of the system. Although the second order enclosure is perhaps the most common type, it is not well suited for protecting the loudspeaker due to its open face.
Alternatively, a fourth order enclosure is better suited for applications in which the loudspeaker must be protected. The fourth order enclosure is commonly called a bandpass enclosure because it is designed to have a large acoustic output over a narrow frequency range. The primary enclosure parameters which determine the operational frequency range and the amount of sound produced are the back volume, front volume, port area, and port length. By varying the values and ratios of these parameters, the sound level and bandwidth of the enclosure can be altered to meet the desired output requirements.
In view of these characteristics, the fourth order enclosure is generally preferred for ANC muffler applications for cancelling engine exhaust. The fourth order enclosure is particularly well suited for these applications because the partially enclosed front volume mechanically protects the loudspeaker cone from intruding objects and the outside elements. Furthermore, the fact that a fourth order enclosure system produces high sound energy in a small frequency band is ideal for low frequency engine exhaust tones.
The most widely utilized method for securing a loudspeaker within an enclosure is to use multiple fasteners, typically four to eight, around the loudspeaker flange. With fourth order enclosures, this mounting method is commonly facilitated by creating a back volume cover for enclosing the back of the loudspeaker and attaching to the front portion of the enclosure, thus creating a seal. This two-piece design is sometimes referred to as a "clam shell" design. However, this design has several drawbacks including an increased number of parts to manufacture, and difficulty in servicing once mounted under the vehicle body. Moreover, the prior designs require a large number of fasteners which in turn necessitates additional drilling and tapping processes which are expensive. Additional problems with mounting the speaker via the mounting flange are installing and removing the fasteners within a small enclosure area, and non-uniform loudspeaker gasket compression. More specifically, this configuration causes stress concentrations in that the loudspeaker gasket naturally compresses more around the screws and less in the area between the screws, thus creating a sealing problem.
An additional problem recognized within the art is thermal failure of the loudspeaker voice coil due to overheating. Often times, the requirements for a smaller enclosure require using a smaller loudspeaker which can sacrifice acoustic performance. To compensate for this loss in performance, the loudspeakers are driven at higher levels, often approaching or exceeding their upper limits. At higher input levels, the magnet and voice coil of the loudspeaker heats up and under extreme conditions can overheat and become damaged or destroyed. Accordingly, it is desirable to provide an enclosure and system for mounting the loudspeaker which provides heat sinking to remove excess heat from the loudspeaker magnet and voice coil.
One of the challenges presented to further development of ANC muffler systems is designing a suitable enclosure which is lightweight, efficient to assemble in a mass production environment, and which is unaffected by its operating environment including water, high levels of heat, and stones thrown under the vehicle. In view of the limitations associated with the prior art, it is desirable to provide an enclosure and mounting apparatus which provides a simple and cost efficient apparatus for mounting the loudspeaker within the enclosure. It is further desirable to provide an enclosure and mounting apparatus which allows the loudspeaker to be replaced by removing a minimal number of fasteners using simple hand operated tools. Finally, it is desirable to provide a mounting apparatus which also provides for the dissipation of heat generated by the loudspeaker during operation.